Mark Zuckerberg cofounded Facebook in his Harvard dorm room in 2004. By 2007, he was a millionaire. One year later Facebook debuted on the New York Stock Exchange as the biggest tech IPO in history, minting the world's youngest self-made billionaire at the time: 23-year-old Zuckerberg.

Berkshire Hathaway founder Warren Buffett's long-game investing strategies finally cemented his billionaire status at 56 — about 26 years after he first joined the seven-figure club. At age 87, he's now worth nearly $85 billion.

Amazon founder Jeff Bezos was a millionaire for two years before he became a billionaire in 1998, when Amazon went public. At the time, Buffett commanded a fortune of nearly $30 billion. Nearly 20 years later, Bezos is about $30 billion richer than Buffett.

Check out the graphic below to see how long it took the richest people in the world to go from millionaire to billionaire. The list excludes two people — Françoise Bettencourt-Meyers and Mukesh Ambani — who both inherited their billions, skipping the millionaire stage completely. All current net worth figures are from Forbes' real-time ranking as of April 4.